1. Field of the Invention
This invention relates, in general, to electrical insulating bushings and, more specifically, to electrical insulating bushings of the fluid-filled type.
2. Description of the Prior Art
Electrical insulating bushings are utilized to connect electrical apparatus, such as transformers, circuit breakers and the like, to an electrical circuit. The current rating of conventional bushings, especially those used in high current applications, is directly proportional to the quantity of heat that is dissipated from the current carrying conductor. In order to prevent excessive temperature rise in the conductor during current flow and also to minimize the size of the bushing itself, cooling means are used to dissipate the heat generated by the current carrying members of the bushing.
A typical approach is shown in U.S. Pat. No. 2,859,271, issued to D. Johnston et al, and U.S. Pat. No. 1,878,094, to R. Atkinson, wherein an insulative tubular sleeve is radially spaced from the conductor such that an annular duct is formed therebetween. The duct is disposed in fluid communication with an outer channel formed between the insulative sleeve and the outer housing of the bushing. Dielectric fluid, such as oil, flows through the duct by a thermosiphon action and thereby dissipates heat from the central conductor. The quantity of heat dissipated by such a bushing is limited since only the surface of the central conductor exposed to the duct is cooled by the oil.
U.S. Pat. No. 3,626,079, issued to Keen and Lynch, discloses a similar bushing construction in which the current carrying conductor is formed of a pair of concentric tubular electrodes connected together with an annular duct therebetween, which in turn is radially spaced around a hollow insulative shell. Oil is pumped from an oil-filled electrical apparatus through the hollow shell and, by a series of baffles and apertures, flows along the outer surface of the outer electrode, through the annular duct between the two electrodes and across the inner surface of the inner electrode to the oil-filled tank. Although such a construction exposes the entire surface of both conductors to the dielectric fluid, a pump is required to force the oil through the ducts. A similar approach is shown in U.S. Pat No. 1,706,810, issued to W. Paul, wherein a coolant fluid, in this case air, flows down through a hollow central conductor and up through an annular duct formed between the outer surface of the conductor and the insulative housing of the bushing. In these types of bushings, the coolant fluid flows serially across the surface of the conductors, that is, the same fluid flows across both the inner and outer surfaces of the conductor. The heat dissipation capability of a given quantity of dielectric fluid progressively decreases in such a bushing since the temperature of the fluid increases as it flows across the conductor surface which reduces the amount of heat that can be dissipated by a given quantity of dielectric fluid and thus lowers the current carrying capability of the bushing.
Furthermore, it is necessary to provide means to prevent the leakage of coolant from fluid-filled bushings caused by the differences in thermal expansion of the central conductor and the exterior bushing housing. A typical approach, as disclosed in U.S. Pat. Nos. 2,853,538, 2,933,551 and 3,178,504, all assigned to the assignee of the present invention, utilizes an expansion cap surrounding the central conductor of the bushing and having a plurality of compression springs disposed therein which exert tensile stress on the conductor and compressive stress on the exterior bushing housing to ensure an oil-tight seal over the entire operating range of the bushing. Although the expansion cap provides adequate sealing for the bushing, it does not provide additional cooling for the oil contained within the housing and, in particular, for the portion of the conductor contained within the expansion cap which can become excessively hot during current flow. Thus, it would be desirable to provide an electrical insulating bushing for use in high current applications which has a higher current carrying capability than that attainable in prior art insulating bushings due to improved cooling techniques. It would also be desirable to provide a new and improved insulating bushing wherein efficient cooling of the current carrying members is achieved by thermosiphon flow thereby eliminating the need for forced coolant flow by means of mechanical pumps.